1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly, to an implenting in-plane switching (IPS) liquid crystal display device.
2. Discussion of the Related Art
In general, a liquid crystal display device makes use of optical anisotropy and polarization properties of liquid crystal molecules to produce an image, wherein the liquid crystal molecules have an initial alignment direction due to their long, thin shape. In addition, the alignment direction can be controlled by inducement of an electric field, wherein the alignment direction of the liquid crystal molecules changes as the induced electric field changes. Thus, refraction of incident light is dependent upon the alignment direction of the liquid crystal molecules due to the optical anisotropy properties of the liquid crystal molecules. Therefore, images can be produced by properly controlling the induced electric field.
Of the different types of known liquid crystal display (LCD) devices, active matrix LCD (AM-LCDs) devices, which have a matrix configuration of thin film transistors (TFTs) and pixel electrodes, are presently being development because of their high resolution and superiority in displaying moving images. The LCD devices have wide application in office automation (OA) equipment and video devices because of their light weight, thin profiles, and low power consumption characteristics. Commonly, a liquid crystal display panel has an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. The upper substrate is commonly referred to as a color filter substrate, and usually includes a common electrode and color filters. The lower substrate is commonly referred to as an array substrate, and includes switching elements, such as thin film transistors and pixel electrodes.
As previously described, when the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. Thus, the liquid crystal molecules function as an optical modulation element having variable optical characteristics that are dependent upon a polarity of a voltage applied to the common electrode and the pixel electrode. Accordingly, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the LCD devices having longitudinal electric fields are disadvantageous since they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD device commonly includes a lower substrate, which has a pixel electrode and a common electrode disposed thereon, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates.
FIG. 1 is a schematic cross sectional view of an IPS-LCD panel according to the related art. In FIG. 1, upper and lower substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 40 is interposed therebetween. On the lower substrate 20 are a common electrode 36 and a pixel electrode 38, wherein the common and pixel electrodes 36 and 38 are aligned parallel to each other. On a surface of the upper substrate 10, a color filter layer (not shown) is positioned corresponding to an interval between the pixel electrode 38 and the common electrode 36 of the lower substrate 20. A voltage applied across the common and pixel electrodes 36 and 38 produces an electric field 21 through the liquid crystal layer 40. Since the liquid crystal layer 40 has a positive dielectric anisotropy, liquid crystal molecules of the liquid crystal layer 40 align parallel to the electric field 21.
FIGS. 2A and 2B are schematic cross sectional views of an IPS-LCD device according to the related art. As shown in FIG. 2A, when no electric field is produced by the common and pixel electrodes 36 and 24, i.e., an OFF state, longitudinal axes of liquid crystal (LC) molecules 42 are parallel and form a definite angle with the common and pixel electrodes 36 and 38. For example, the longitudinal axes of the LC molecules 42 are arranged parallel with both the common and pixel electrodes 36 and 38.
Conversely, as shown in FIG. 2B, when a voltage is applied to the common and pixel electrodes 36 and 38, i.e., an ON state, an in-plane electric field 21 parallel to the surface of the lower substrate 20 is produced because the common and pixel electrodes 36 and 38 are on the lower substrate 20. Accordingly, the LC molecules 40 are re-aligned having their longitudinal axes coincidence with (parallel to) the electric field. However, the first LC molecules 40a positioned directly adjacent to the common and pixel electrodes 36 and 38 do not change their alignment, while the second LC molecules 40b positioned between the common and pixel electrodes 36 and 38 are aligned perpendicular to the common and pixel electrodes 36 and 38. Thus, a wide viewing angle is produced ranging from about 80 to 85 degrees along vertical and horizontal directions from a line vertical to the IPS-LCD panel. However, if the common and pixel electrodes 36 and 38 have parallel rectilinear shapes, the IPS-LCD panel produces a color shift phenomenon and/or a gray scale inversion. Thus, the common and pixel electrodes are formed to have a zigzag shape, as shown in FIG. 3.
FIG. 3 is a plan view of an array substrate for an IPS-LCD device according to the related art. In FIG. 3, a gate line 12 including at least one gate electrode 14 is formed along a first direction on a substrate 20, wherein the gate electrode 14 protrudes from the gate line 12. In addition, a data line 24 is disposed substantially perpendicular to the gate line 12 along a second direction, and a common line 16 is arranged parallel with the gate line 12. Accordingly, the gate line 12, the common line 16, and a pair of the data lines 24 define a pixel region P on the substrate 20. Furthermore, an island-shaped semiconductor layer 22 is positioned near the crossing of the gate and data lines 12 and 24, thereby forming a thin film transistor (TFT) T having a source electrode 26 and a drain electrode 28. The gate electrode 14 protruded from the gate line 12 is disposed between the source and drain electrodes 26 and 28, and corresponds to the island-shaped semiconductor layer 22.
As shown in FIG. 3, a plurality of pixel electrodes 30b are disposed within the pixel region P, wherein the plurality of pixel electrodes 30b extend from a pixel connector 30a disposed to overlap the common line 16. In addition, a plurality of common electrodes 17 extending from the common line 16 are also disposed within the pixel region P, wherein each of the common electrodes 17 corresponds to the pixel electrode 30b and also has a zigzag shape. Thus, liquid crystals 63, which have an initial direction in accordance with a rubbing direction, are oriented along electric fields K when electric signals are supplied to the zigzag-shaped common and pixel electrodes 17 and 30b. 
In the array substrate shown in FIG. 3, since the common and pixel electrodes 17 and 30b have the same zigzag shape, the pixel region P has multiple domains therein. Accordingly, the liquid crystals in one domain have a symmetrical figure arranged symmetrically to the liquid crystals in second, adjacent domains when the electric fields K are generated. The multiple domain structures described above minimize the color shift phenomenon and suppress the gray scale inversion due to the multiple domains offsetting birefringence within the pixel regions P.
However, the IPS-LCD device shown in FIG. 3 is problematic. For example, since the common and pixel electrodes 17 and 30b have the zigzag shapes, the liquid crystals 63 located near bent portions C of the common and pixel electrodes 17 and 30b are not oriented although the electric signals are supplied to the common and pixel electrodes 17 and 30b. This is due to the electric field generated near the bent portions C being perpendicular to the initial alignment direction formed by rubbing processes. Furthermore, the liquid crystals located in left and right portions D1 and D2 of the pixel region P do not properly respond to the electric field due to the fact that the electric fields generated in the left and right portions D1 and D2 are not suitable enough to twist the liquid crystals 63. For example, the directions of electric fields in the left and right portions D1 and D2 are relatively abnormal as compared with those generated between the common and pixel electrodes 17 and 30b. Therefore, light leakage may occur in the portions D1 and D2, and the disclination may be caused by misalignment of the liquid crystals 63. In other IPS-LCD devices, a black matrix is adopted in order to cloak the disclination and the light leakage, but this may decrease aperture ratios of the IPS-LCD devices.